Nicene and Ante-Nicene Fathers, Ser. II, Vol. XI:The Works of John Cassian.: Chapter XVI. How what is good in its own nature can be distinguished from other things that are good.

Chapter XVI.

How what is good in its own nature can be distinguished from other things that are good.

And this may be distinguished from those other things which we have termed indifferent, in these ways: if a thing is good in itself and not by reason of something else: if it is useful for its own sake, and not for the sake of something else: if it is unchangeably and at all times good, and always keeps its character and can never become anything different: if its removal or cessation cannot fail to produce the greatest harm: if that which is its opposite is in the same way evil in its own nature, and can never be turned into anything good. And these descriptions by which the nature of things that are good in themselves can be distinguished, cannot possibly be applied to fasting, for it is not good of itself, nor useful for its own sake because it is wisely used for the acquisition of purity of heart and body, that the pricks of the flesh being dulled the soul may be pacified and reconciled to its Creator, nor is it unchangeably and at all times good, because often we are not injured by its intermission, and indeed sometimes if it is unreasonably practised it becomes injurious. Nor is that which seems its opposite evil in its own nature, i.e., the partaking of food, which is naturally agreeable, which cannot be regarded as evil, unless intemperance and luxury or some other faults are the result; “For not that which entereth into the mouth, defileth a man, but that which cometh out of the mouth, that defileth a man.” 2191
And so a man disparages what is good in its own nature, and does not treat it properly or without sin, if he does it not for its own sake but for the sake of something else, for everything else should be done for the sake of it, but it should be sought for its own sake alone.